There has been known a diffraction lattice of a phase grating type being comprised of many micro unit patterns which are arranged regularly in a rectangular or triangular lattice and repeatedly at a pitch of 10 to 30 .mu.m, each of the unit patterns being comprised of one or more projections and one or more recessions which are so formed as to have a predetermined phase difference therebetween against a light having a desirable wave length ranging from 400 to 700 nm and each of the projections or recessions being formed into a circle, a ring or a combination thereof.
A diffraction lattice of this type is especially suitable for a focusing plate for detecting a focusing state of a focusing lens assembly, since the diffracted light of zero order can be controlled in strength thereof, and at the same time, the diffraction or scattering character of the lattice can be controlled (see, for instance, JP-A No. 11931/1980 or No. 41621/1982).
Generally speaking, features required for a focusing plate of an optical system are as follows;
(A) The image projected thereon can be observed brightly and clearly,
(B) When the image projected thereon is in a defocus condition, such image can be observed in a naturally blurred state.
(C) When the focusing plate is used in a view finder, it should not be observed as coarse by grained representation.
In order to satisfy these features A, B and C favorable for a focusing plate, the diffraction lattice of the phase grating type should have characteristics as follows;
(a) In order to satisfy the feature A, almost of all the diffracted light should be included within a maximum angle which is determined according to the F-number of the focusing lens assembly. In other words, almost of all the light having been passed through the lens assembly can be observed on the focusing plate.
(b) In order to satisfy the feature B, the image appearing on the focusing plate is made blurred as a whole and uniformly in a defocus condition by heightening the maximum order of the diffracted light to be included within the maximum angle as high as possible and by equalizing each of intensities of diffracted light of respective orders as far as possible. Conversely, it is impossible to obtain the image blurred naturally as mentioned above in the case wherein the diffracted light of specific orders having intensities stronger than those of others are enhanced.
(c) The coarse grained image as mentioned in the item (C) is directed to such images as observed when frosted glass is used for a focusing plate. As for a bi-level focusing plate of a phase grating type, each of the unit patterns formed thereon is observed as a grained image, when the pitch thereof is increased to a degree of the resolving power of the human eye. Accordingly, it is important to make the size or the pitch of the unit pattern as small as possible in order to eliminate such grained images. In this respect, these are observed wherein the pitch of the unit pattern exceeds 30 .mu.m in the case of view finder installed in an ordinary single lens reflex camera. Also, each of the unit patterns will be observed as a grained image in the view finder of a video camera unless the repeated pitch is set smaller than 20 .mu.m, since the video camera has a magnification higher than that of the single lens reflex camera.
Although it is desirable to make the size of the unit patterns as small as possible in order to avoid the grained image of the focusing plate, this makes the manufacturing thereof extremely difficult.
As is disclosed in the Japanese references cited above, it is necessary, in order to satisfy the characteristic (b), to make the diffusion or diffraction pattern flat by making the configuration of projections or recessions in each of the unit patterns complicated and/or miniatured as far as possible.
FIG. 1 shows a unit pattern of a square configuration as is disclosed in JP-A No. 41621/1982. The unit has one circular projection of a large radius disposed at the center thereof, four circular projections of a medium radius disposed at each corner thereof and four circular projections of a small size disposed between adjacent two projections. If unit patterns are designed so as to have a pitch of 20 .mu.m, the diameter of the small circular projection will be of an order of 2.4 .mu.m and the minimum distance among these projections will be of an order of 1.6 .mu.m. As is known to those skilled in the art, the method applicable for forming these micro unit patterns is a photo-etching as is used for forming a pattern of an integrated circuit of a semiconductor. Even if the photo-etching method is employed, it becomes considerably difficult to form such complicated unit patterns with such a narrow distance as mentioned above. This causes the cost for manufacturing the focusing plate to increase to an unprofitable level.